1. Field of the Invention
The present invention relates to a method for regulating the disalt content in .alpha.-sulfofatty acid ester surfactants in the process for the manufacture thereof.
2. Description of Related Art
Wash-active substances based on .alpha.-sulfofatty acid esters or their salts have been known for decades and numerous processes have been proposed for their production (see e.g. U.S. Pat. Nos. 2,195,187, 3,256,303 and 3,158,632). The salts of .alpha.-sulfofatty acid esters acting as wash-active substances are obtained by sulfonation of lower alkyl esters of saturated higher fatty acids with sulfur trioxide. In particular, fatty acid methyl esters which contain from 6 to 28 carbon atoms in the fatty acid residue and which, apart from the CH.sub.2 -group in the .alpha.-position of the fatty acid residue, contain no other sulfonatable or sulfatable groups and which have an iodine number below 5, are sulfonated with a sulfur trioxide-inert gas mixture and the reaction product is neutralized. Since dark-colored crude products regularly accumulate during the sulfonation reaction, being unsuitable for use in detergents and cleaners in that form, the crude sulfonation product has to be bleached. H.sub.2 O.sub.2 and/or hypochlorite in aqueous solution is/are normally used for bleaching.
It is also known that considerable quantities of disalt of the corresponding .alpha.-sulfofatty acids accumulate as an undesirable secondary product during this sulfonation of fatty acid esters and during work-up of the .alpha.-sulfofatty acid ester crude product with aqueous media. These disalts of the .alpha.-sulfofatty acids are undesirable for several reasons. First of all, they show only limited solubility in water. Secondly, they exhibit poor surface activities. Above all, however, they deteriously influence the viscosity of the aqueous ester sulfonate pastes ultimately produced. An excessive content of the disalts accumulating as secondary product leads to a considerable increase in the viscosity of the aqueous ester sulfonate pastes which in turn gives rise to difficulties during further processing of the ester sulfonate surfactant.
Considerable attention has been devoted, especially in recent years, to this particular aspect of the production of surfactants based on ester sulfonates. Numerous difficulties result, cf. U.S. Pat. No. 4,404,143 and German Offenlegungsschrift No. 33 34 517. According to the first of these two publications, a highly concentrated aqueous solution of a salt of .alpha.-sulfofatty acid esters is prepared by neutralizing the sulfonated fatty acid ester product with an aqueous alkali solution in two stages, in the first of which the sulfonated product is neutralized to a pH-value of from 2.5 to 4 with an aqueous alkali solution of relatively high concentration (15 to 50% by weight of alkali) in the presence of a C.sub.1 -C.sub.4 alcohol in a quantity of from 5 to 20% by weight, based on the weight of the sulfonated product, after which neutralization is completed to a pH-value of from 6 to 7 in a second stage carried out with a less concentrated aqueous alkali solution. The crude sulfonation product may optionally be bleached before this two-stage neutralization. To this end, an aqueous solution of H.sub.2 O.sub.2 is preferably used, again in the presence of a C.sub.1 -C.sub.4 alcohol. The hydrogen peroxide is supposed to be used in the form of an aqueous solution having a concentration of 10% by weight or higher. The preferred alcohol is methanol where the fatty acid esters are methyl esters. Utilizing this technique is said to reduce the disalt content of the corresponding .alpha.-sulfofatty acids to 5% or lower.
However, the second of the two above-mentioned publications, German Application No. 33 34 517, describes the disadvantages of this process. The sulfonation products obtained contain the short-chain alcohol used in a large excess in the aqueous neutralized reaction product. These comparatively large quantities of free alcohol are again undesirable for a number of reasons. They are troublesome, for example, during the work-up of surfactant mixtures of the above type in the production of detergent mixtures by spray-drying, particularly giving rise to undesirable pluming. In addition, the free alcohols present in the surfactant mixture have an unpleasant odor which necessitates deodorization. To solve these problems, German Offenlegungsschrift 33 34 517 proposes carrying out the aqueous bleaching and neutralization of the crude .alpha.-sulfofatty acid esters in the presence of such quantities of a lower alcohol that an aqueous suspension containing from 30 to 55% by weight of the .alpha.-sulfofatty acid ester salt and, based on the weight of the .alpha.-sulfofatty acid ester salt, from 5 to 15% by weight of a lower alcohol sulfate and from 8 to 40% by weight of the lower alcohol is obtained. Finally, the aqueous suspension is said to be concentrated in such a way that it contains from 40 to 65% by weight of .alpha.-sulfofatty acid ester salt, from 2 to 10% by weight of a lower alcohol sulfate and at most 2% by weight of a lower alcohol.